Known hydraulic drive systems equipped on hydraulic machines such as hydraulic excavators and cranes are described in, e.g., JP-A-3-213703, JP-A-7-63203 and JP-A-1-312201.
The hydraulic drive system described in JP-A-3-213703 comprises a variable displacement hydraulic pump, directional control valves of center bypass type for controlling flows of a hydraulic fluid supplied to a plurality of actuators from the hydraulic pump, and a pump control device for controlling the delivery rate of the hydraulic pump to become a flow rate corresponding to shift amounts by which the directional control valves are operated. The directional control valves of center bypass type include each a throttle (center bypass throttle) in its center bypass passage. Downstream of the center bypass throttle, there is provided a pressure compensating valve for controlling the differential pressure across the center bypass throttle to be kept constant.
The hydraulic drive system described in JP-A-7-63203 comprises a variable displacement hydraulic pump, a plurality of actuators driven by a hydraulic fluid delivered from the hydraulic pump, a plurality of directional control valves of closed center type for controlling flows of the hydraulic fluid supplied to the plurality of actuators, a plurality of control lever units for operating the plurality of directional control valves, a bypass line connected to a delivery line of the hydraulic pump, a bleed valve disposed in the bypass line and returning the hydraulic fluid delivered from the hydraulic pump to a reservoir when the plurality of directional control valves are in their neutral positions, and a bleed control device for controlling the bleed valve so that the bleed valve has an opening corresponding to input amounts by which the plurality of control lever units are operated.
The hydraulic drive system described in JP-A-1-312201 is constructed as shown in FIG. 15.
In FIG. 15, a valve apparatus comprising pressure compensating valves 82A, 82B, variable throttle valves 80A, 80B of closed center type, and directional control valves 81A, 81B is connected to a supply line 3 of a hydraulic fluid delivered from a variable displacement pump 1. Actuators 6, 7 are connected respectively to the directional control valves 81A, 81B through load lines 81Aa, 81Ab and load lines 81Ba, 81Bb. Also, the variable throttle valves 80A, 80B and the directional control valves 81A, 81B are driven for operation by respective pilot pressures produced by control lever units 30A, 30B.
Lines 83A, 83B for detecting load pressures of the associated actuators are connected respectively to lines interconnecting the variable throttle valves 80A, 80B and the directional control valves 81A, 81B. The detected load pressures are introduced as control signals to the pressure compensating valves 82A, 82B, and the detecting lines 83A, 83B are connected to a shuttle valve 84. Maximum one of the load pressures of the actuators 6, 7 driven by the hydraulic pump 1 is detected through the shuttle valve 84 and introduced to a maximum load pressure detecting line 85b.
Further, in a bypass line 5 branched from the supply line 3 of the hydraulic pump 1, there are disposed an unloading valve 85 to which the delivery pressure of the hydraulic pump 1 and the detected maximum load pressure are introduced respectively through signal lines 85a, 85b and which drains a part of the flow delivered from the hydraulic pump 1 when the differential pressure between the pump delivery pressure and the maximum load pressure exceeds the pressure difference preset by a spring 85s, and a pressure generator, downstream of the unloading valve 85, comprising a throttle 42 and a relief valve 43. A pressure generated by the pressure generator is introduced to a tilting control device 2n of the hydraulic pump 1 through a signal line 44 to carry out negative flow control under in such a manner that the delivery rate of the hydraulic pump 1 is decreased or increased in accordance with an increase or decrease of the pressure generated by the pressure generator depending on an increase or decrease of the amount by which the hydraulic fluid is drained through the unloading valve 85.